If you provide your code, it will be easier to find the problem. One thought, based on a recent similar error, though basic, is did you include "import arcpy" as the first line in the code window?
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Get SpatialMay 8 '12 at 4:18

Yes, I just got an email from ESRI support confirming that there is a bug and the licensing issue should be resolved in SP4. Alternatively, I have written a python function that uses arcpy.AddField_management() and arcpy.UpdateCursor() to emulate JoinField_management() without updating.
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AlexMay 14 '12 at 1:26

@AlexOlssen, wow, talk about forgetting a basic question. Asking about the ArcGIS version is great, but the service pack is also critical! Glad you found a solution!
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Get SpatialMay 29 '12 at 0:29

Hi, I am not at work and don't have access to ArcGIS right now. However, I have the spatial analyst license, and the code works perfectly without the last line. The file sum_slope.dbf is correctly created and can be loaded into ArcMap. Thus I don't know which parameter would not be valid. I will check the link you supplied first thing tomorrow. Thank you.
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AlexMay 8 '12 at 6:38

@AlexOlssen, I modified my answer. It looks like you were referencing a different function in your script than the help file that you linked to. Hopefully this will work when you get back to the office.
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Get SpatialMay 8 '12 at 7:17

Hi, now it is my turn to apologize! I have tried JoinField_management() and AddJoin_management() because the former wouldn't work. In an earlier version I used the exact code you suggested; I have just rechecked it and I get the same error. Moreover, in ArcMap, if I go to the "geoprocessing" drop down menu, select "search for tools", type "join field (data management)" and click the "toolboxes\system toolboxes\..." link, then the tool appears with a lock next to it. :(
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AlexMay 9 '12 at 3:32

That page is for ArcGIS 9.3. @Djmapper gave the correct answer; I got the same thing from ESRI. JoinField_management should be in all licenses of ArcGIS 10, however in SP1 and SP2 there is a bug with the licensing. If you upgrade to SP3 or SP4 then everything works fine.
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AlexMay 25 '12 at 23:17